The species might have been a komodo dragon, or something much largerRyan Vaarsi/Flickr

The earliest humans to live in Australia had to content with giant killer lizards, very possibly from the extinct Megalania monitor lizard species, which could grow up to 6m in length and weighed 500kg.

Scientists at the University of Queensland found a fossilised bone from a giant lizard during an excavation in Central Queensland. The bone is that of a Komodo dragon or a much larger species, like Megalania.

Paeleocologist Dr Gilbert Price said: "Our jaws dropped when we found a tiny fossil from a giant lizard during a 2m-deep excavation in one of the Capricorn Caves, near Rockhampton. The 1cm bone, an osteoderm, came from under the lizard's skin and is the youngest record of a giant lizard on the entire continent.

"We can't tell if the bone is from a Komodo dragon – which once roamed Australia – or an even bigger species like the extinct Megalania monitor lizard, which weighed about 500kg and grew up to 6m-long."

The tiny bone shows giant killer lizards lived in Australia at the same time as humansGilbert Price

The first humans and these giant lizards would have overlapped territories around 50,000 years ago. "The find is pretty significant, especially for the timeframe that it dates," he said.

As well as massive lizards, early humans would have had to put up with 9m-long inland crocodiles that also roamed Australia in the last Ice Age in the Pleistocene period. The latest lizard bone was found at the Capricorn Caves, one of Australia's most fossil-rich site.

Researchers do not know how the lizard ended up in the cave, but said the find highlights how much more they have to find at the site. Price also said the lizards might have been hunted by humans, possibly driving them to extinction. "It's been long-debated whether or not humans or climate change knocked off the giant lizards, alongside the rest of the megafauna," he said. "Humans can only now be considered as potential drivers of their extinction."